1. Field of the Invention
This invention relates to lens systems and, in particular, to lens systems having a wide field of view and/or telecentric properties. Among other applications, such systems can be used as projection lenses for projection televisions.
2. Description of the Prior Art
In general terms, the field of view of a lens system determines what proportion of the surface of an extended object can be imaged by the system. More precisely, the field of view is defined as the angle which the entrance window subtends at the center of the entrance pupil, where the entrance window and the entrance pupil are, respectively, the images of the field stop and the aperture stop in object space, i.e., the images of those stops by those elements of the system which precede the stop.
Lens systems having a field of view greater than about 20-25 degrees (half or semi-field) are generally considered to be wide angle systems. Such systems are used extensively in projecting, recording, and copying applications where a product having a compact size is desired. In particular, projection television systems normally employ a projection lens having a wide field of view.
Telecentric lens systems are defined in terms of the location of the system's exit and/or entrance pupils or equivalently in terms of principal rays. As indicated above, the entrance pupil of a lens system is the image of the system's aperture stop in object space. Similarly, the exit pupil is the image of the aperture stop in image space. That is, the entrance pupil is the image of the aperture stop produced by the elements of the system which precede the aperture stop, and the exit pupil is the image produced by the elements which follow the aperture stop. The principal ray emanating from an off-axis object point is that ray which passes through the middle of the aperture stop. Since the exit and entrance pupils are images of the aperture stop, the principal ray also intersects the axis of the optical system at the locations of the pupils in the absence of aberrations.
A telecentric lens system is a lens system which has at least one of its pupils at infinity. In terms of principal rays, having a pupil at infinity means that the principal rays are parallel to the optical axis a) in object space, if the entrance pupil is at infinity, or b) in image space, if the exit pupil is at infinity. Since light can propagate through the lens system in either direction, the pupil at infinity can serve as either an entrance or an exit pupil depending upon the system's orientation with respect to the object and the image. Accordingly, the term "telecentric pupil" will be used herein to describe the system's pupil at infinity, whether that pupil is functioning as an entrance or an exit pupil.
In practical applications, the telecentric pupil need not actually be at infinity since a lens system having an entrance or exit pupil at a sufficiently large distance from the system's optical surfaces will in essence operate as a telecentric system. The principal rays for such a system will be substantially parallel to the optical axis and thus the system will in general be functionally equivalent to a system for which the theoretical (Gaussian) location of the pupil is at infinity. Accordingly, as used herein, the term "telecentric lens system" is intended to include lens systems which have at least one pupil at a long distance from the lens elements, and the term "telecentric pupil" is used to describe such a pupil at a long distance from the lens elements.
Optical systems having telecentric properties have typically been used in applications where a beam of light needs to be perpendicular to an object plane or an image plane. For example, in measurement (metrology) applications such as contour projectors used to check the dimensions of small mechanical parts, systems having telecentric entrance pupils are often used since measurements made in the image plane for such systems are relatively insensitive to defocussing of the system. See, for example, Hirose, U.S. Pat. No. 4,511,223, and Miyamae et al., U.S. Pat. No. 4,637,690. Similarly, telecentric systems have been used in laser beam scanners and in fiber optic systems where images are relayed through multiple fiber bundles. See, for example, Shirota, U.S. Pat. No. 4,925,279, and EPO Patent Publication No. 373,677.
Telecentric lens systems have also been used in the field of projection television and, in particular, for projection television systems using liquid crystal displays (LCDs) as light valves. Such systems can be of the transmissive type where light comes in from behind the liquid crystal panel and is modulated as it passes through the panel or of the reflective type where light enters through the front of the panel and is reflected back out towards the screen after having been modulated. See, for example, Taylor, U.S. Pat. No. 4,189,211, Gagnon et al., U.S. Pat. No. 4,425,028, Gagnon, U.S. Pat. No. 4,461,542, Ledebuhr, U.S. Pat. No. 4,826,311, Minefuji, U.S. Pat. No. 4,913,540, EPO Patent Publication No. 311,116, and Russian Patent Publication No. 1,007,068.
In either case, the modulated light leaving the panel for the screen tends to be oriented perpendicular to the face of the panel, e.g., the cone of light leaving each pixel of the panel has a half angle on the order of 10.degree.. In contrast, the light leaving the phosphor screen of a cathode ray tube propagates in all directions, i.e., the screen functions as a Lambertian source. Accordingly, lens systems designed for use with cathode ray tubes are not particularly efficient when used with a liquid crystal panel. Telecentric systems, on the other hand, with their parallel principal rays are well-suited for gathering in the quasi-parallel light leaving the face of a liquid crystal panel and transferring that light to an image screen.
In view of their numerous applications, a wide variety of designs have been developed for telecentric lenses. See, for example, Ikemori, U.S. Pat. No. 3,947,094, Tateoka, U.S. Pat. No. 4,441,792, and Russian Patent Publications Nos. 603,938, 1,048,444, and 1,089,535. Generally, these designs have suffered from a number of drawbacks. For example, in order to keep the aperture stop at a place where it will be imaged at a long distance from the system's optical elements, prior art telecentric lens systems have typically had a limited aperture and field of view. Also, lens systems of this type have tended to be large in size and complicated in construction.